College students have been heading back to school, and so has President Obama. For the last two weeks, the president has been visiting campuses in swing states around the country. He's been urging students to register and vote. His campaign says it is also working to win the votes of young people who are not in school. Here's NPR's Scott Horsley.

SCOTT HORSLEY, BYLINE: It's a brand new semester on college campuses, and that means a brand new class of potential voters. Rebecca Hinch attended an Obama rally this week near the University of Virginia.

REBECCA HINCH: All of my friends are here. So the majority of people at least I know are here for Obama. So I think it's definitely, he's a young people's person.

HORSLEY: Four years ago, Mr. Obama won more than two-thirds of the youth vote, and he still enjoys a sizeable lead over Mitt Romney and his iPod-loving running mate Paul Ryan. Still, a survey this summer by Circle, a Tufts-University center that studies young people and politics, found nearly four in 10 voters under the age of 30 are disappointed with Mr. Obama. For some, the change he promised has not come quickly enough.

PATRICK MORGAN: Well, four years is a long time for me.

HORSLEY: Patrick Morgan of Falls Church, Virginia is 18 years old. That means he was just 14 when the Obama presidency began.

MORGAN: I wouldn't say I'm disappointed. It didn't live up to my hopes. But it didn't fall below my expectations. I think there's some things that I wish could've been done that haven't, closing of Guantanamo Bay and wish there could've been a lot more done with immigration and solved that.

HORSLEY: At another campus rally in Iowa this week, Mr. Obama warned his political opponents will try to capitalize on youthful disillusionment.

(SOUNDBITE OF SPEECH)

PRESIDENT BARACK OBAMA: They're counting on young people sitting this one out. They say, well, you know? Obama's, you know, he's greyer now. He's not as new and as fresh as he was in 2008, so young people aren't going to turn out the same way.

HORSLEY: The president sounds as if he's joking, but his concern is genuine. Republicans are highlighting the high unemployment rate among young people: 13-and-a-half percent for those between 20 and 25. What's more, many young people now take for granted changes that seem dramatic to their parents - on gay rights, for example.

(SOUNDBITE OF SPEECH)

OBAMA: We don't need to rewrite our Constitution to somehow say that people who love each other and aren't bothering anybody else, that somehow they cannot get married.

HORSLEY: Young people may applaud the president's newfound endorsement of same-sex marriage. But to those like Rebecca Hinch, Mr. Obama's personal evolution seems less like breaking ground than catching up.

HINCH: I mean, I have a lot of gay friends at UVA, so I just think it's dumb that we're still arguing over it, honestly, and that it's such a big issue. It should be, like, you know, not a big deal. That's my personal thing.

HORSLEY: Each year, Beloit College prepares a kind of field guide to update professors on how incoming freshmen see the world. This year's guide notes that for young people just entering college, Bill Clinton is an elder statesman. Richard Nixon has always been dead, and the secretary of state has almost always been a woman.

OLIVIA BROWN: It's been most of my life like that. And looking back at how it used to be, I think that shows that it would be, like, progress that our country's going through, being able to have so much diversity in our government.

HORSLEY: But for UVA sophomore Olivia Brown, that kind of diversity, even the first African-American president, is something she's more or less grown accustomed to. So Mr. Obama takes pains to remind young people of other changes that have come in the last four years, including the health care overhaul, the end of the war in Iraq and big investments in green energy. The president acknowledges there's still a lot of unfinished work to do. As he did in 2008, he flatters young people about their role in making it happen.

(SOUNDBITE OF SPEECH)

OBAMA: I'm asking you to believe, not in my ability to bring about the changes you want to see. I'm asking you to believe in your abilities. I'm asking you to believe in what you can accomplish.

HORSLEY: That message works for Olivia Brown.

BROWN: I know a lot of people are probably - didn't get all the change that they wanted. But I don't think that kind of change can come in just four years. And I'm really excited to be able to spend my first vote on voting for him.

HORSLEY: Some 15 million Americans have turned 18 and become eligible to vote since Mr. Obama's first election four years ago. He hopes a lot of them feel the same way.